Document webpack change: improve merging of resolve and parsing options

A pull request by @sokra was merged and maintainers requested a documentation change.

See pull request: https://github.com/webpack/webpack/pull/9422

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by rules and via loader API

Arrays overwrite by default, but can reference old value with "..."

partial backport of https://github.com/webpack/webpack/pull/9126

What kind of change does this PR introduce? feature

Did you add tests for your changes? yes

Does this PR introduce a breaking change? no

What needs to be documented once your changes are merged? Merging of Rule.resolve/Rule.parser objects has a new feature: In array the special "..." item can now be used to reference to old value, while otherwise array values overwrite each other. This also affects the getResolve(options) API in the loader API